Method of developing a nickel-base superalloy

ABSTRACT

The invention relates to a method of developing a nickel-base superalloy consisting of a γ-phase and γ′-phase for the production of single-crystal or directionally solidified bodies of material. The invention is characterized in that the properties of nickel-base superalloys with a volume proportion of γ′-phase of at least 50% after a degradation at room temperature are optimized, in that the composition of the alloy is chosen such that at room temperature a lattice displacement (δ) between the γ-phase and the γ′-phase is as high as possible. It is thereby attained that the yield strength at room temperature after degrading is comparatively high, and thus only a small difference of the yield strengths occurs between initial state and degraded state.

[0001] This application is a Continuation of and claims priority under 35 U.S.C. § 120 to International application number PCT/IB02/04619, filed 05 Nov. 2002, and claims priority under 35 U.S.C. 119 to Swiss application number 2001 2059/01, filed 09 Nov. 2001, the entireties of both of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a method of developing a nickel-base superalloy which is used for the production of a single-crystal or directionally solidified body of material.

[0004] 2. Brief Description of the Related Art

[0005] A whole series of nickel-base superalloys are known from the prior art, and are used for the production of single-crystal or directionally solidified bodies of material. Such bodies of material are used, for example, in power station construction with high temperature loading. Material strength at high temperature can, for example, be maximized by means of these single crystal components, whereby in its turn the inlet temperature of gas turbines can be increased, which leads to an increase of efficiency of the gas turbine.

[0006] Heretofore such alloys were developed according to the following concepts:

[0007] increase of creep strength,

[0008] increase of oxidation- and corrosion-resistance,

[0009] increase of resistance to crack growth, particularly to LCF (low cycle fatigue),

[0010] improvement of castability and of heat treatment possibilities,

[0011] reduction of costs.

[0012] Known nickel-base superalloys are, for example, the alloys CMSX-2, CMSX-4, CMSX-10, Rene N5, Rene N6, PWA 1484 and PWA 1483, and their combination, for example, can be gathered from G. L. Erickson: Corrosion Resistant Single Crystal Superalloys for Industrial Gas Turbine Application, International Gas & Turbine Aeroengine Congress & Exhibition, Orlando, Fla., Jun. 2-Jun. 5, 1997. Such alloys are subjected after the casting process to a heat treatment in which in a first solution annealing step the γ′-phase unevenly precipitated during the casting process is completely or partially dissolved. In a second heat treatment step this phase is precipitated again in a controlled manner. To attain optimum properties, this precipitation heat treatment is carried out such that fine uniformly distributed particles of the γ′-phase arise in the γ-phase.

[0013] It is known that lattice displacement can play a decisive role for creep strength at high temperatures. Many of the known nickel-base superalloys have a positive or negative lattice displacement between the γ-matrix and the γ′-phase. Dislocations upon sliding or cutting of γ′ grains are prevented by this lattice distortion, which effects an increase of the short time strength at elevated temperatures. While on the one hand in the literature on nickel-base superalloys at room temperature a negative lattice displacement with the highest possible amplitude is required (P. Caron: High γ′ solvus new generation nickel-based superalloys for single crystal turbine blade applications. Proceedings of the 9th international symposium on superalloys—SUPERALLOY 2000, pp. 737-746, Champion, USA, Sep. 17-21, 2000), other nickel-base superalloys (see, for example, EP 0 914 483 B1) are designed by a corresponding choice of the addition elements such that no lattice displacement is present, since it was observed that a directed grain enlargement of the γ′-particles, and following this a degradation of the γ′-structure, arises over a long period at high temperatures due to a lattice displacement between γ- and γ′-phases in the presence of a moderate or low mechanical stress.

SUMMARY OF THE INVENTION

[0014] The invention seeks to avoid the disadvantages of the known prior art, and has as its object to provide a method for developing nickel-base superalloys which depends on a new, simple concept.

[0015] The object is attained according to the invention in that the properties of nickel-base superalloys with a volume fraction of less than 50% after degradation at room temperature are optimized, in that the composition of the alloy is chosen such that at room temperature a lattice displacement 6 between the γ-phase and the γ′-phase is as high as possible, where δ[%]=2 (a_(γ′)−a_(γ))/(a_(γ′)+a_(γ)) and a_(γ)is the lattice constant of the γ-phase and a_(γ′) is the lattice constant of the γ′-phase.

[0016] The advantages of the invention consist in that it is relatively easy to develop nickel-base superalloys having optimized degradation behavior with this method.

[0017] It was found that in the presence of a mechanical load and a long-time high temperature stress, a directed coarsening of the γ′-particles, the so-called raft formation (rafting) occurs and, at high γ′-content (i.e., with a γ′ volume fraction of at least 50%), tends toward inversion of the microstructure, i.e., the γ′ becomes the continuous phase, in which the earlier γ-matrix is embedded. Since the intermetallic γ′-phase tends toward environmental embrittlement, under certain load conditions this leads to a large decrease in the mechanical properties, above all of the yield stress, at room temperature. The environmental embrittlement particularly occurs when moisture and long holding times under tensile load are present. If according to the invention a high positive lattice displacement between the γ-phase and the γ′-phase is selected, the degradation of the properties is then less strongly marked, i.e., the loss of yield strength in the degraded state, compared with the undegraded state, is only small.

[0018] It is advantageous if the lattice constants of the γ-phase a_(γ) and of the γ′-phase a_(γ′) are determined according to the following known equations: $\begin{matrix} {{a_{\gamma}\lbrack Å\rbrack} = {3.524 + {0.0196\quad {Co}} + {0.110\quad {Cr}} + {0.478\quad {Mo}} + {0.444\quad W} +}} \\ {{{0.441\quad {Re}} + {0.3125\quad {Ru}} + {0.179\quad {Al}} + {0.422\quad {Ti}} +}} \\ {{{{0.7\quad {Ta}} + {0.7\quad {Nb}}},}} \end{matrix}$

[0019] where the numbers before the element symbols give the relative atomic fraction of the respective element in the γ-phase and $\begin{matrix} {{a_{\gamma^{\prime}}\lbrack Å\rbrack} = {3.57 - {0.004\quad {Cr}} + {0.208\quad {Mo}} + {0.194\quad W} + {0.262\quad {Re}} +}} \\ {{{{0.1335\quad {Ru}} + {0.258\quad {Ti}} + {0.5\quad {Ta}} + {0.46\quad {Nb}}},}} \end{matrix}$

[0020] where the numbers before the element symbols give the relative atomic fraction of the respective element in the γ′-phase.

[0021] A degradation parameter D is now introduced for characterizing the creep behavior of nickel-base alloys, and is determined according to the following equation:

D=(T−800)t ^(1/2)σ^(1/5)

[0022] with T=temperature in ° K, t=time in h, and σ=stress in MPa. The yield stress σ_(0.2) of nickel-base alloys at room temperature in the degraded state is determined based on the degradation parameter, and those alloys are chosen which have the smallest differences of the yield stress between the initial state and the degraded state, i.e., those alloys which have the highest possible values of the yield stress in the degraded state

[0023] Still other objects, features, and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of embodiments constructed in accordance therewith, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiment examples of the invention are shown in the diagrams.

[0025]FIG. 1 shows the dependence of yield stress after degradation at room temperature of the lattice displacement between the γ-phase and the γ′-phase for various known nickel-base superalloys, and

[0026]FIG. 2 shows the dependence of the yield stress at room temperature on the degradation parameter for various known nickel-base superalloys.

[0027] Only the features essential for the invention are shown. Like elements have the same reference numerals in both Figures

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The invention is explained in detail hereinafter, using embodiment examples and the accompanying FIGS. 1 and 2.

[0029] It was found that with prior application of a mechanical load and long term high temperature stress, a directed enlargement of the γ′-particles, so-called raft formation (rafting), occurs, and that with the presence of a high γ′-content (i.e., with a γ′-volume content of at least 50%), inversion of the microstructure occurs, i.e., γ′ becomes the continuous phase, and is embedded in the previous γ-matrix. Since the intermetallic γ′-phase tends toward environmental embrittlement, under given load conditions this leads to a moderate decrease of the mechanical properties, above all of the yield strength at room temperature. Thus a degradation of the properties results. Environmental embrittlement particularly occurs when tensile loading occurs for long holding times in the presence of moisture. If according to the invention an alloy with a high positive lattice displacement δ between the γ-phase and the γ′-phase is now chosen, the degradation of properties is less strongly marked, i.e., the loss of yield strength in the degraded state is only small in comparison with the undegraded state.

[0030]FIG. 1 shows, for different known nickel-base superalloys which are used for producing a single-crystal or directionally solidified workpiece body, the dependence of the yield strength σ_(0.2) after degradation at room temperature of the lattice displacement δ between the γ-phase and the γ-phase. The lattice displacement δ between the γ-phase and the γ′-phase was calculated in the known manner as follows:

δ[%]=2(a _(γ′) −a _(γ))/(a _(γ′) +a _(γ))

[0031] where a_(γ) is the lattice constant of the γ-phase and a_(γ′) is the lattice constant of the γ′-phase.

[0032] Alloys with the chemical composition set out in Table 1 (data in wt. %) were used.

[0033] The lattice displacement δ between the γ-phase and the γ′-phase at room temperature in the alloys investigated is located in the region of about −0.24% to +0.58%. With increase of the positive lattice displacement, the yield strength σ_(0.2) also increases after degradation at room temperature. Of the alloys investigated, the alloy PWA1480 has the highest positive lattice displacement δ between the γ-phase and the γ′-phase, and consequently also the highest yield strength σ_(0.2) after degradation at room temperature.

[0034] The lattice constants of the γ-phase a_(γ) and of the γ′-phase a_(γ′) were determined according to the following, known per se (see P. Caron: High γ′ solvus new generation nickel-based superalloys for single crystal turbine blade applications. Proceedings of the 9th international symposium on superalloys—SUPERALLOY 2000, pp. 737-746, Champion, USA, Sep. 17-21, 2000): $\begin{matrix} {{a_{\gamma}\lbrack Å\rbrack} = {3.524 + {0.0196\quad {Co}} + {0.110\quad {Cr}} + {0.478\quad {Mo}} + {0.444\quad W} +}} \\ {{{0.441\quad {Re}} + {0.3125\quad {Ru}} + {0.179\quad {Al}} + {0.422\quad {Ti}} +}} \\ {{{{0.7\quad {Ta}} + {0.7\quad {Nb}}},}} \end{matrix}$

[0035] where the numbers before the element symbols give the relative atomic fraction of the respective element in the γ-phase and $\begin{matrix} {{a_{\gamma^{\prime}}\lbrack Å\rbrack} = {3.57 - {0.004\quad {Cr}} + {0.208\quad {Mo}} + {0.194\quad W} + {0.262\quad {Re}} +}} \\ {{{{0.1335\quad {Ru}} + {0.258\quad {Ti}} + {0.5\quad {Ta}} + {0.46\quad {Nb}}},}} \end{matrix}$

[0036] where the numbers before the element symbols give the relative atomic fraction of the respective element in the γ′-phase. The alloying elements B, Zr and C play no significant part in relation to the lattice displacement, especially as they are only present as trace elements in small quantities.

[0037] The degradation behavior of the alloy can now be optimized according to the invention, in that positive lattice displacement δ between the γ-phase and the γ′-phase is set as high as possible by variation of the composition. For characterizing creep behavior, a degradation parameter D was introduced for the nickel-base alloys, and is determined according to the following equation:

D=(T−800)t ^(1/2)σ^(1/5)

[0038] with T=temperature in ° K, t=time in h, and σ=stress in MPa.

[0039] The yield strength σ_(0.2) at room temperature after degrading is then determined based on the said degradation parameter. These values are plotted against each other in FIG. 2 for the alloys from Table 1. In order to optimize properties, the yield strength at room temperature is to be as high as possible for the various degradation parameters. This prescription is best fulfilled by the alloy PW1480, which at room temperature has a lattice displacement δ between the γ-phase and the γ′-phase of +0.58%. The alloy CMSX4, which has a lattice displacement δ between the γ-phase and the γ′-phase of −0.24% at the most in accordance with the invention, but has, on the contrary, based on the degradation parameter D, which is at least about 5,000 KhMPa, the smallest value of the yield strength. This alloy would thus be unsuitable in view of degradation behavior. TABLE 1 Chemical Composition of the Alloys in Wt. %. ALLOY Ni Co Cr Al Ti Mo W Ta Nb Hf B Zr C Re SXIN738 Remainder 8.5 16 3.4 3.4 1.7 2.6 1.7 0.9 — — — 0.03 — SXCM247 Remainder 9.2 8.1 5.6 0.7 0.5 9.5 3.2 — 1.4 0.015 0.015 0.07 — MC2 Remainder 5 8 5.0 1.5 2 8 6 — — — — — CMSX2 Remainder 5 8 5.6 1.0 0.6 8 6 — — — — — — CMSX4 Remainder 9 6.5 5.6 1.0 0.6 6 6.5 — 0.1 — — — 3 CMSX6 Remainder 5 10 4.8 4.7 3 — 2 — 0.1 — — — — PW1480 Remainder 5 10 5.0 1.5 — 4 12 — — — — — — SXN5 Remainder 8 7 6.2 — 2 5 7 — 0.2 — — — 3

[0040] List of Reference Symbols σ_(0.2) yield strength δ lattice displacement a_(γ) lattice constant of the γ-phase a_(γ′) lattice constant of the γ′-phase D degradation parameter T temperature t time

[0041] While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents is incorporated by reference herein in its entirety. 

What is claimed is:
 1. A method of developing a nickel-base superalloy consisting of γ- and γ′-phases, for the production of single crystal or directionally solidified bodies of material, wherein the properties of nickel-base superalloys with a volume fraction of γ′-phase of at least 50% after degrading at room temperature are optimized, comprising: selecting the composition of the alloy such that at room temperature a lattice displacement (δ) between the γ-phase and the γ′-phase is as high as possible, where δ[%]=2(a _(γ′) −a _(γ))/(a _(γ′) +a _(γ)), wherein a_(γ) is the lattice constant of the γ-phase, and wherein a_(γ′) is the lattice constant of the γ′-phase.
 2. A method according to claim 1, comprising: determining the lattice constant (a_(γ)) of the γ-phase and the lattice constant (a_(γ′)) of the γ′-phase according to the following equations: $\begin{matrix} {{a_{\gamma}\lbrack Å\rbrack} = {3.524 + {0.0196\quad {Co}} + {0.110\quad {Cr}} + {0.478\quad {Mo}} + {0.444\quad W} +}} \\ {{{0.441\quad {Re}} + {0.3125\quad {Ru}} + {0.179\quad {Al}} + {0.422\quad {Ti}} +}} \\ {{{{0.7\quad {Ta}} + {0.7\quad {Nb}}},}} \end{matrix}$

wherein the numbers before the element symbols give the relative atomic fraction of the respective element in the γ-phase; and $\begin{matrix} {{a_{\gamma^{\prime}}\lbrack Å\rbrack} = {3.57 - {0.004\quad {Cr}} + {0.208\quad {Mo}} + {0.194\quad W} + {0.262\quad {Re}} +}} \\ {{{{0.1335\quad {Ru}} + {0.258\quad {Ti}} + {0.5\quad {Ta}} + {0.46\quad {Nb}}},}} \end{matrix}$

wherein the numbers before the element symbols give the relative atomic fraction of the respective element in the γ′-phase.
 3. A method according to claim 1, comprising: characterizing the long term behavior of nickel-base superalloys including determining a degradation parameter (D) from the following equation: D=(T−800)t ^(1/2)σ^(1/5) wherein T=temperature in ° K, t=time in h, and σ=stress in MPa; and determining the yield strength (σ_(0.2)) at room temperature after degrading is determined based on said degradation parameter (D).
 4. A method according to claim 3, comprising: maximizing yield strength (σ_(0.2)) at room temperature. 